.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                               1993 Michael Haardt, Ian Jackson.
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
.\"   <michael@cantor.informatik.rwth-aachen.de>
.\" Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aeb@cwi.nl>
.\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
.\" Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
.\"   <michael@cantor.informatik.rwth-aachen.de>
.\"
.TH READ 2 2009-02-23 "Linux" "Linux Programmer's Manual"
.SH NAME
read \- read from a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
.fi
.SH DESCRIPTION
.BR read ()
attempts to read up to
.I count
bytes from file descriptor
.I fd
into the buffer starting at
.IR buf .
.PP
If
.I count
is zero,
.BR read ()
returns zero and has no other results.
If
.I count
is greater than
.BR SSIZE_MAX ,
the result is unspecified.
.SH "RETURN VALUE"
On success, the number of bytes read is returned (zero indicates end of
file), and the file position is advanced by this number.
It is not an error if this number is smaller than the number of bytes
requested; this may happen for example because fewer bytes are actually
available right now (maybe because we were close to end-of-file, or
because we are reading from a pipe, or from a terminal), or because
.BR read ()
was interrupted by a signal.
On error, \-1 is returned, and
.I errno
is set appropriately.
In this case it is left unspecified whether
the file position (if any) changes.
.SH ERRORS
.TP
.B EAGAIN
The file descriptor
.I fd
refers to a file other than a socket and has been marked non-blocking
.RB ( O_NONBLOCK ),
and the read would block.
.TP
.BR EAGAIN " or " EWOULDBLOCK
.\" Actually EAGAIN on Linux
The file descriptor
.I fd
refers to a socket and has been marked non-blocking
.RB ( O_NONBLOCK ),
and the read would block.
POSIX.1-2001 allows either error to be returned for this case,
and does not require these constants to have the same value,
so a portable application should check for both possibilities.
.TP
.B EBADF
.I fd
is not a valid file descriptor or is not open for reading.
.TP
.B EFAULT
.I buf
is outside your accessible address space.
.TP
.B EINTR
The call was interrupted by a signal before any data was read; see
.BR signal (7).
.TP
.B EINVAL
.I fd
is attached to an object which is unsuitable for reading;
or the file was opened with the
.B O_DIRECT
flag, and either the address specified in
.IR buf ,
the value specified in
.IR count ,
or the current file offset is not suitably aligned.
.TP
.B EINVAL
.I fd
was created via a call to
.BR timerfd_create (2)
and the wrong size buffer was given to
.BR read ();
see
.BR timerfd_create (2)
for further information.
.TP
.B EIO
I/O error.
This will happen for example when the process is in a
background process group, tries to read from its controlling tty,
and either it is ignoring or blocking
.B SIGTTIN
or its process group
is orphaned.
It may also occur when there is a low-level I/O error
while reading from a disk or tape.
.TP
.B EISDIR
.I fd
refers to a directory.
.PP
Other errors may occur, depending on the object connected to
.IR fd .
POSIX allows a
.BR read ()
that is interrupted after reading some data
to return \-1 (with
.I errno
set to
.BR EINTR )
or to return the number of bytes already read.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
On NFS file systems, reading small amounts of data will only update the
timestamp the first time, subsequent calls may not do so.
This is caused
by client side attribute caching, because most if not all NFS clients
leave st_atime (last file access time)
updates to the server and client side reads satisfied from the
client's cache will not cause st_atime updates on the server as there are no
server side reads.
Unix semantics can be obtained by disabling client
side attribute caching, but in most situations this will substantially
increase server load and decrease performance.
.PP
Many file systems and disks were considered to be fast enough that the
implementation of
.B O_NONBLOCK
was deemed unnecessary.
So,
.B O_NONBLOCK
may not be available on files
and/or disks.
.SH "SEE ALSO"
.BR close (2),
.BR fcntl (2),
.BR ioctl (2),
.BR lseek (2),
.BR open (2),
.BR pread (2),
.BR readdir (2),
.BR readlink (2),
.BR readv (2),
.BR select (2),
.BR write (2),
.BR fread (3)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
